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The resistant elms
The resistance mechanisms of elms against DED try to restrict the establishment and spread of the pathogen within the vascular system. This resistance is based on different factors, some still not fully understood to science.
In addition to the mentioned factors dissuading bark beetles there is a host genetic resistance, if the pathogen is unable to silence or reprogram resistance genes of the tree, or an environmental resistance, if trees tolerate the disease in the field due to the influence of external factors. For instance, drought can modify the structure of the wood, thus affecting the expansion of the pathogen inside the tree, and soil application of certain phenolic compounds increases tolerance of elms to the pathogen. The study of the microbiota associated with elms will probably increase the knowledge about environmental factors involved in resistance.
Elms show juvenile resistance to Dutch elm disease, so it is not reliable testing the resistance of elms until they are four years old, which slow down the selection and breeding of resistant elms.
For finding elm trees with genetic resistance, firstly those elms that have survived DED epidemics, especially large specimens, are located. Once genetically analyzed, they are in vitro propagated and planted in experimental plots. The resistance to DED is evaluated through artificial inoculation with the fungus. Then, the leaf wilting is monitored during the season.
In this way, seven Ulmus minor clones tolerant to DED were obtained. They have been registered by the Spanish Environmental Administration as qualified forest reproduction material. This allows the use of these clones for forest purposes.
Currently, research is going on for obtaining new tolerant clones and hybrids between them, to increase their tolerance.
After 30 years of work and cooperation between the Environmental Administration and the School of Forest Engineers (Technical University of Madrid), in January 2014 an important milestone was reached with the inclusion of 7 Ulmus minor clones tolerant to DED in the National Catalog of Forest Reproductive Materials.
Here, the main features of these clones, especially the mean values of foliar wilting after inoculation, are shown and compared to those of a resistant control clone (SAPPORO) and a susceptible clone (M- DV1 or Commelin).